1. Field of the Invention
The invention relates to a device for flexibly coupling two rotary shafts, and more particularly, though not exclusively, to such devices which are used in aeronautics and which usually include a flexible metallic member rigidly connected to each of the shafts.
Flexible couplings are well known for rotatably connecting two shafts which can suffer a misalignment of their respective axes or a divergence in the positioning of the shafts along their axes. The flexible couplings used in aeronautics must be capable of performing well in a small overall space, with a low misalignment of the shafts and a low axial offset of these shafts. Such couplings, which may have a diameter of less than 100 mm, generally transmit a torque of 1000 Nm at speeds of 20,000 rpm and temperatures reaching 100.degree. C., and must possess total reliability over a specific working life. Good behaviour under vibration is also necessary. This implies:
a high degree of rigidity during rotation, so as not to promote the development of oscillations in the rotational movement; and,
a reduction in oscillation of the flexible parts of the coupling in directions parallel to the axis of rotation.
2. Summary of the Prior Art
Such couplings usually include a metallic flexible member, means for connecting the flexible member to the two shafts, and means for reducing vibrations in the coupling.
When the coupling is to operate in both directions of rotation, the flexible member includes deformable elements which are subjected to traction in one direction of rotation and compression in the other, and other deformable elements acting in an opposite manner.
In French Patent No. 2 409 417 a coupling device is disclosed in which the flexible member comprises a stack of spring steel leaves which are held spaced from one another and are rigidly bolted to two plates at the end of each shaft. In a first embodiment, the leaves are generally in the shape of a regular convex polygon, and are spaced apart by washers. In a second embodiment, the leaves are in the form of segments arranged along the sides of a regular polygon and bolted at their ends to the two plates, the leaves of one side being spaced apart by having the leaves of the two adjacent sides interleaved between them. In both cases, a damping material is provided between the leaves at the spaces where the latter are likely to vibrate, so as to reduce these fatigue generating vibrations, as well as the mutual friction resulting therefrom, such friction causing wear of the leaves.
One of the problems associated with such devices is the concentration of stresses at the edges of the connection zones of the flexible member, this concentration reducing the resistance of the coupling to breakage and fatigue. This effect is increased as a result of the faces of the deformable elements being at a sharp angle to the edge of the means which keeps them apart, whether this be washers or other deformable elements.
Another problem which tends to occur when the deformable elements are subjected to compression is the development of an instability deformation termed "buckling". It is well known that buckling imposes very substantial stresses on couplings, and experience has shown that breaks are most likely to occur in deformable elements when they are in compression. The deformable elements must be thin to restrict the stresses occurring as a consequence of misalignment of the shafts, but this thinness makes them more vulnerable to buckling.
The occurrence of buckling deformation is also promoted by:
bending of the deformable elements resulting from misalignment of the shafts; and,
the vibrations and resonances to which the flexible elements are subjected when these elements bend as a consequence of the misalignment of the shafts occurring in alternation.
Another problem stems from the cantilever mounting of the flexible member which is bolted to the plates parallel to the axis of the shafts. This arrangement creates a lever effect sensitive to the forces of the transmission torque which are perpendicular to the axis of the shafts, this lever effect itself causing twisting of the connecting means and an additional deformation of the flexible member causing further stress and increased risks of buckling.
German Patent No. 2 048 152 discloses a flexible coupling comprising a unitary disk-shaped flexible member having a plurality of radial slots forming the resilient leaves. The flexible member is made of rubber-like material or some other suitable non-metallic material moulded over bushes through which pins can pass to enable a connection to be made to the shafts. The flexible member as described and shown in the drawings requires a very flexible material such as rubber, and cannot be used for the problem to be solved by the present invention, as it would not withstand the high temperatures to which it would be exposed and it would not possess sufficient rigidity during rotation for the high torques to be transmitted.
European Patent No. 0 059 267 discloses a flexible coupling in which the flexible member is constituted by leaves made of fiber-reinforced synthetic material and moulded together so as to form a unitary flexible member. Such a flexible member cannot be used to solve the present problem for the same reasons as apply to the above-mentioned German patent. Moreover, the flat arrangement of the reinforcing fibers does not enable the latter to oppose the stress concentrations on the surface of the leaves at the edges of the connection zone. The synthetic material consequently has a tendency to form cracks in this area, which makes the coupling fragile and shortens its life.